CCD imagers in general include a plurality of photodetectors arranged in rows and columns and CCD shift registers arranged between the rows of the photodetectors. The photodetectors in each row are coupled, such as by a transfer gate, to their adjacent CCD shift register so that the charge carriers accumulated in the photodetectors can periodically be transferred to the CCD shift register. The CCD shift register transfers the charge carriers to the read-out of the device. It has been found that if a photodetector accumulates an excessive amount of the charge carriers prior to their being transferred to the CCD shift register, some of the charge carriers will overflow from the photodetector into the CCD shift register and/or adjacent photosites. This adversely affects the charge carriers in the shift register being transferred to the read-out structure. This effect is referred to as "blooming".
Conventional techniques for preventing blooming, i.e. anti-blooming techniques, use an overflow drain between the photodetector and the adjacent CCD shift register with the drain being isolated from the photodetector by a potential barrier. The height of the barrier is controlled either by a diffusion or by a separate gate. During integration, the charge in the photodetector builds up as a result of the impinging radiation. If the charge level in the photodetector reaches a sufficient amount to raise the photodetector potential to a level above that of the barrier between the photodetector and the overflow drain, additional signal carriers will be swept over into the anti-blooming drain where they are removed by the drain supply. This prevents excess charge from flowing into the CCD shift register and thereby prevents blooming. During this integration period, the charge in the photodetector is isolated from the CCD shift register by a transfer gate which is held at a potential high enough to form a barrier to the carriers higher than the anti-blooming drain barrier. During the transfer period, the transfer gate barrier is lowered allowing the charge to flow from the photodetector into the CCD shift register. However, a problem with this anti-blooming structure may arise under high exposure conditions. During the transfer operation, there is not only a flow of the charge from the photodetector to the CCD shift register, but also a flow of any excess charge directly into the CCD shift register. If the total amount of charge is greater than the charge capacity of the shift register cell, the result is a similar effect as if blooming had occurred from the photodetector itself during the integration stage. Therefore, it is desirable to have an anti-blooming structure which prevents blooming not only during the integration stage, but also during the transfer stage.